1. Field of the Invention
This invention relates to sinterable compositions of fluorite oxides for making ceramic bodies, and to a method of making ceramic bodies from such compositions. The fluorite oxides include zirconia ZrO.sub.2, ceria CeO.sub.2 and thoria ThO.sub.2, and, for the purposes of this specification, include solid solutions between two or more fluorite oxides, for example zirconia containing a minor proportion of hafnia (HfO.sub.2), a common impurity in solid solution.
2. Description of the Prior Art
Zirconia has attracted attention as an ionic conductor which may be used in oxygen monitors, fuel cells and batteries. The ionic conductivity is typically conferred by a 12 mol % substitution of zirconium oxide by calcium oxide CaO (lime), yielding a defect structure permitting oxygen-ion diffusion, and also serving to suppress the monoclinic-to-tetragonal phase transformation which pure zirconia undergoes on heating to about 1200 C. and which can lead to disintegration of a zirconia object.
Other zirconia compositions known as ionic conductors are ZrO.sub.2 containing 8 mol % yttria Y.sub.2 O.sub.3 (more expensive, but displaying good conductivity and stability against ageing), ZrO.sub.2 containing scandia Sc.sub.2 O.sub.3 (even more expensive, but having excellent conductivity) and ZrO.sub.2 containing rare earth oxides. Zirconia may also be stabilised by mixtures of such materials. Zirconia containing 12 mol % CaO or 8 mol % Y.sub.2 O.sub.3, wherein the 1200 C. phase transformation is regarded as fully suppressed, is accordingly known as `stabilised zirconia`. Smaller amounts of CaO, e.g. 6 mol %, do not fully suppress the transformation, and such compositions are described as `partially stabilised`.
Other fluorite oxide compositions known as good ionic conductors are ceria containing gadolinia or yttria, and thoria containing yttria. These have cubic structures under all conditions, and the above considerations about `stabilisation` do not arise.
All these compositions share the problem that they are difficult to sinter.
Sintering methods which are available include hot pressing, which is limited to certain shapes and is expensive. Special coprecipitated powders and special techniques such as hydrothermal in situ oxidation or hot isostatic pressing may be used, but increase the cost substantially. Liquid-phase additives are a common expedient, e.g. (for zirconia with lime) 2 mol % alumina Al.sub.2 O.sub.3 or 5 mol % titania TiO.sub.2, but have a significantly damaging effect on the conductivity of the zirconia, and (since they act by forming boundary films around the zirconia particles) increase grain boundary creep at high temperatures. Therefore this method, while economical and useful for various applications, is disadvantageous where good mechanical properties at high temperatures or good ionic conductivity are important.
The best sintering method in theory would use a solid-state additive, i.e. an additive which goes into solid solution in the host. In such a method, no liquid is formed during sintering and the creep strength, conductivity and stability of the material are unaffected. For solid-state sintering of alumina, for example, it is known to add 1/4 weight % nickel oxide NiO or 1/4 weight % magnesia MgO. Such systems are not however easy to find, and none has been known, so far as we are aware, for stabilised zirconia.
Solid-state additives have commonly been claimed to work by influencing the defect structure of the host, whereby vacancies or interstitials are created in the host lattice, permitting rapid diffusion and hence sintering. However, stabilised zirconia already has high defect concentrations, from (for example) 12 mol % CaO creating vacancies, yet it still sinters poorly. This instance shows that experience with other solid-state additive systems does not help us in selecting a system for zirconia.